1. Field of the Invention
The present invention discloses a power supply and a power controlling method thereof, and more particularly, to a power controller operated with light-load or no load and a power controlling method thereof.
2. Description of the Prior Art
A power supply is utilized for transforming power so as to provide specification-qualified power to electronic devices or elements. Lower power consumption in the power supply is advantageous when considering transformation efficiency. More particularly, when the power supply is under light-load or no-load, tiny increases in power consumption significantly reduce the transformation efficiency of the power supply. Therefore, reducing power consumption of the power supply under light-load or no load conditions is an important goal for designers.
Under light-load or no load conditions, a switched mode power supply is designed to enter a skip mode or a burst mode. Taking flyback power supply 60 shown in FIG. 1 as an example, power management controller 74 is configured to control power switch 72 so as to cause transformer 65 to be energized by input voltage source AC or to de-energize to an output voltage VOUT. Compensation signal SCOM is controlled by the output voltage VOUT via a feedback path formed by inductor LT431 and photo coupler 63. FIG. 2 illustrates inner circuitry of power management controller 74a which can replace power management controller 74 shown in FIG. 1. When burst signal SBST is asserted, indicating a switching state of power supply 60, clock signal SCLK generated by clock generator 86 is utilized for periodically switching on power switch 72 via logic controller 62. The ON time of power switch 72 is controlled by constraint signal SCS-L and comparator 82. Constraint signal SCS-L is generated by level shifter 67 and can be regarded as being equivalent to compensation signal SCOM. Resistor 61 provides a power path, which supplies with power from voltage source VCC so as to generate compensation signal SCOM. When compensation signal SCOM is lower than burst reference signal VBST-REF, comparator 84 disasserts burst signal SBST so that power switch 72 cannot be activated by clock signal SCLK and power supply 60 is kept in a non-switching state. FIG. 3 illustrates waveforms of clock signal SCLK, signal VG, and current detection signal VCS shown in FIG. 2 when power management controller 74a is under light-load or no-load. In response to the variation of compensation signal SCOM, power management controller 74a controls signal VG to switch on and off power switch 72 for one or a plurality of consecutive switch periods, and to constantly turn off power switch 72 for another or a plurality of successive, consecutive switch periods; this indicates a skip mode or a burst mode, and is unified as the burst mode in the following descriptions.
Conceptually, the burst mode is utilized for skipping a plurality of roughly-invalid, consecutive switch periods but focusing power transformation on a plurality of valid consecutive switch periods. However, if the power transformation is not appropriately controlled during the valid consecutive switch periods, audio noises may occur under the burst mode. For example, if group frequency fG corresponding to group period TG, which, as shown in FIG. 3, is the sum of time TB of staying in the switching state and time TS of staying in the non-switching state, falls within an audible frequency range, annoying audio noise is likely to occur.
These and other objectives of the present invention will no doubt become obvious to those of ordinary skill in the art after reading the following detailed description of the preferred embodiment that is illustrated in the various figures and drawings.